The layer two specification of the 3GPP (Third Generation Partnership Project) series of specifications features an Automatic Repeat Request (ARQ) mechanism as part of the Radio Link Control (RLC) protocol. When RLC creates Protocol Data Units (PDU), data is submitted to the lower layer and buffered until its reception is acknowledged by the receiver or the discard timer orders to discard the PDU. A monotonically increasing sequence number is assigned to each PDU allowing the receiver to re-order the stream of received PDUs as well as detect missing packets in the received sequence.
RLC specifies packet formats and procedures to carry buffer status information from the receiver via a STATUS PDU as well as a procedure for the transmitter to request that information, which is performed by setting a specified “Poll” bit in selected RLC PDUs.
Various timers and events trigger the transmission of either a Poll or a STATUS control commands. For instance, the transmitter may send a Poll command at periodic intervals, every time N PDUs are transmitted, or whenever the last data in the RLC buffer is transmitted. The receiver may send a STATUS PDU in response to a Poll command, autonomously at periodic intervals (i.e., every time N PDUs are received), or whenever a missing PDU is detected because of a hole in the sequence numbers.
When the transmitter is to send a Poll, the transmitter selects a RLC PDU that is not yet acknowledged and retransmits the PDU with the Poll bit set to “1” in order to convey the Poll command to the receiver. An example of such an implementation is in Universal Mobile Telecommunication System (UMTS) release 5. Changes in the system architecture of both HSPA+ (High-Speed Packet Access Evolution) and 3GPP LTE (Third Generation Partnership Program Long Term Evolution) are to support larger PDUs in order to reduce the overheads. While retransmitting a complete PDU may have been a satisfactory solution for PDUs of modest size, it may become wasteful to retransmit a large PDU in the newer systems.